The invention relates to an electronic circuit being supplied from differential voltage (e.g. positive and negative) power supply terminals, in which a terminal in the circuit is biased to a voltage in the range between the voltages of the power supply terminals.
The circuit of the invention is i.a. suitable for use in differential crystal oscillators or similar circuits.
Differential crystal oscillators are well known in the art, being supplied with power via resistors or inductors.
From U.S. Pat. No. 5,912,594 a Pierce oscillator is known, having a current source type biasing element, used as an emitter impedance. According to said patent, this element represents a high impedance, ideally a no-load impedance for high-frequency currents.
It is an object of the present invention to improve the general performance of circuits of the type mentioned at the outset, in particular of a low-voltage differential oscillator.
In a first aspect, the present invention provides an electronic circuit comprising first and second power supply terminals having different voltage potentials, a circuit node arranged to be biased to a voltage in the range between the voltages of said first and second power supply terminals by controllable current generator means, wherein the controllable current generator means is arranged to stabilise the voltage of said circuit node by using one or more control signal sources which form part of a closed loop in the electronic circuit.
Two kind of control signal sources are used to stabilise the voltage of the said circuit node; a control signal source where the control signal is derived from the signal to be controlled, e.g. by an AGC (Automatic Gain Control) circuit, and a reference control signal source, e.g. by a DC voltage source. In general, any control/feedback circuit uses both with the circuit configured so that the signal derived from the signal to be controlled is held at the same level as the reference signal. The control signal source is generally a circuit, and the AGC circuit is an example of one such circuit.
Preferably, the first and second power supply terminals respectively have positive and negative voltage potentials.
In an electronic circuit being supplied from positive and negative power supply terminals, a terminal in the circuit being biased to a voltage in the range between the voltages of said positive and negative power supply terminals, this object is met in that connections from said power supply terminals to the circuit are made through current generator means.
By this measure, said connections will attain a comparatively high impedance, seen from the circuit towards the supply lines, and thereby providing an improved isolation of the circuit against small signals from the supply lines (typically noise and stray signals), as well as large signals from the supply lines (deviations of the supply voltage, etc.).
Further, control of the working conditions of the circuit will be facilitated as the current generator means will readily permit biasing the circuit or part of it to a desired voltage between the voltages of the supply lines, the current generator means enabling the circuit to xe2x80x9cfloatxe2x80x9d freely between the voltages of the supply lines.
When controllable current sources or generators are used, the parameters of the circuit may advantageously be adapted to shifting working conditions by suitable control of the currents supplied by the current generators.
The electronic circuit may be a RF (radio frequency) circuit. The electronic circuit may be an oscillator circuit. In such a case, the circuit may be a crystal oscillator circuit. In such circuits, the isolation mentioned will be of particular importance. In RF circuits, said high impedances will facilitate an effective decoupling of the supply lines. In oscillator circuits, an improved isolation is in any case important in order to obtain as low a noise level on the oscillator output signal as possible.
In a particularly preferred embodiment, the circuit comprises a common-base, balanced Pierce crystal oscillator comprising two transistors with their bases connected to a fixed DC bias source, the collectors of the transistors connected to each their output terminal of the circuit and to each their terminal of a quartz crystal, capacitive voltage divider feedback paths leading from each collector to an associated transistor emitter.
Preferably, the electronic circuit is a balanced circuit comprising two substantially identical or mirrored sub-circuits. In another preferred embodiment, the circuit may be a balanced circuit comprising two substantially identical or mirrored sub-circuits and wherein the circuit comprises a balanced Pierce oscillator.
When use is made of the invention in a balanced circuit, a synergetic effect is obtained in that the high impedances of the current generator means tend to enhance the isolation of the circuit from common mode disturbances which is generally already present to a substantial degree in a balanced circuit.
The electronic circuit may comprise two or more controllable current generator means and a corresponding number of circuit nodes, wherein two or more of the current generator means are arranged to utilise the same control signal source to stabilise the voltage in the corresponding circuit node. In this case, particular advantages are obtained in that a circuit parameter for the balanced circuit as a whole will be controllable from one and the same control input terminal. In this way, a particularly efficient and exact control of said common mode voltage is obtained.
In this case, said control signal source may form part of a closed loop stabilising the differential voltage of the nodes connecting said two controllable current generators to said electronic circuit. In this way, a particularly efficient and exact control of said differential voltage is obtained, which according to the invention may advantageously be combined with the aforementioned control of the corresponding common mode voltage.
The control signal source is generally a circuit. The control signal source may be an AGC circuit. In this way, a particularly simple and component effective AGC is had for a differential electronic circuit, in situations where e.g. the gain of the circuit is depending on the current supplied or drained by the current generator means.
In one specific embodiment, the electronic circuit may comprise two or more controllable current generator means and a corresponding number of circuit nodes, wherein two or more of the current generator means are arranged to utilise the same control signal source to stabilise the voltage in the corresponding circuit node and wherein said same control signal source forms part of a closed loop stabilising the common mode voltage of the nodes connecting said two controllable current generators to said electronic circuit.
In one embodiment, the electronic circuit may comprise two or more controllable current generator means and a corresponding number of circuit nodes, and the outputs of the controllable current generator means may each be connected to different nodes, and these nodes may be connected to one another by one or more active/passive components. The active component may be a transistor or other such component, and the passive component may be a resistor or other such component.
In this case, the circuit connected between the outputs of the controllable current generator means will have a comparatively large impedance with respect to supply terminals.
In another specific embodiment, the electronic circuit may comprise a common-base, balanced Pierce crystal oscillator comprising two transistors with their bases connected to a fixed DC bias source, the collectors of the transistors connected to each their output terminal of the circuit and to each their terminal of a quartz crystal, capacitive voltage divider feedback paths leading from each collector to an associated transistor emitter, characterised in that:
current is supplied from a first power supply terminal to the transistor collector terminals via controllable current generators controlled from a servo loop circuit having the common mode voltage of said transistor collector terminals as the controlled variable;
current is added to the currents supplied from the first power supply terminal to said collector terminals by controllable current generators controlled from differential outputs of a differential amplifier in a servo loop circuit having the differential voltage of said transistor collector terminals as the controlled variable; and
current is drained from the transistor emitters to a second power supply terminal via controllable current generators controlled from an AGC circuit controlling the output voltage and/or the gain of the crystal oscillator.
In this circuit, current is according to the invention supplied from a first power supply terminal to the transistor collector terminals via controllable current generators controlled from a servo loop circuit having the common mode voltage of said transistor collector terminals as the controlled variable; current is added to the currents supplied from the first power supply terminal to said collector terminals by controllable current generators controlled from differential outputs of a differential amplifier in a servo loop circuit having the differential voltage of said transistor collector terminals as the controlled variable; and current is drained from the transistor emitters to a second power supply terminal via controllable current generators controlled from an AGC circuit controlling the output voltage and/or the gain of the crystal oscillator.
This circuit combines the advantages of the invention mentioned above, the invention thereby providing a balanced Pierce type oscillator of superior performance, thereby contributing i.a. to: enhanced isolation from supply lines, improved control of crystal oscillating mode, independent control or servo control of differential and common mode working points, simple but effective AGC means, and use of supply voltages down to 1.8 V.
In such a case, the controllable current generators may comprise, as a major functional element, a FET (field effect transistor). A FET considered as a component has as intrinsic characteristics many of the features needed in a controllable current generator. Thus, a considerable saving in component count may be obtained by utilising in the circuit of the invention the rather linear interdependence between gate voltage and drain current of the FET.
In other embodiment, the controllable current generators may comprise, as a major functional element, a FET (field effect transistor) and wherein controllable current generators are substantially constituted by each one MOS-FET (metal oxide semiconductor field effect transistor), the source and drain of said FET constituting the supply terminals of the respective current generator, and the gate of said FET constituting the control input terminal of said respective current generator.
This embodiment will imply an ultimately simple controllable current generator, having quite satisfactory performance for the purpose of this invention.